Condenser microphones generally have a movable diaphragm that vibrates to produce a signal representative of an incident audio signal. To ensure that audio signals contact their respective diaphragms, prior art condenser microphones known to the inventors have apertures in their backplates directly under a solid portion of the diaphragm. Accordingly, audio signals pass through the backplate apertures to directly contact the diaphragm.
Condenser microphones typically are responsive to audio signals having frequencies that are greater than a predetermined low frequency cutoff point. This low frequency cutoff point often is set by controlling the resistance of the air flowing past the microphone diaphragm. This resistance, however, can be relatively high due to the positioning of the apertures directly under a solid portion of the diaphragm. Undesirably, setting the low frequency cutoff can be difficult due to such high resistance.
One method of controlling this low frequency cutoff point/resistance varies the gap formed between the diaphragm and the stationary support structure supporting the diaphragm. For example, the gap may be enlarged to raise the cutoff point, or reduced to lower the cutoff point. Such a method, however, has drawbacks. Among other things, it dictates the gap size in a manner that may interfere with other design considerations.
In addition, controlling the gap size often does not sufficiently address the above noted air resistance problem, in which the backplate aperture is directly under a solid portion of the diaphragm. Specifically, a portion of the sound wave path must be generally horizontal to reach the diaphragm gap. As such, controlling the gap size provides relatively coarse control of the cutoff point. Electronic or other non-mechanical means then may be required to sufficiently tune the cutoff point of the microphone.